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Solar fuel generation from thermochemical H2O or CO2 splitting is a promising and attractive approach for harvesting fuel without CO2 emissions. Yet, low conversion and high reaction temperature restrict its application. One method of increasing conversion at a lower temperature is to implement oxygen permeable membranes (OPM) into a membrane reactor configuration. This allows for the selective separation of generated oxygen and causes a forward shift in the equilibrium of H2O or CO2 splitting reactions. In this research, solar-driven fuel production via H2O or CO2 splitting with an OPM reactor is modeled in isothermal operation, with an emphasis on the calculation of the theoretical thermodynamic efficiency of the system. In addition to the energy required for the high temperature of the reaction, the energy required for maintaining low oxygen permeate pressure for oxygen removal has a large influence on the overall thermodynamic efficiency. The theoretical first-law thermodynamic efficiency is calculated using separation exergy, an electrochemical O2 pump, and a vacuum pump, which shows a maximum efficiency of 63.8%, 61.7%, and 8.00% for H2O splitting, respectively, and 63.6%, 61.5%, and 16.7% for CO2 splitting, respectively, in a temperature range of 800 °C to 2000 °C. The theoretical second-law thermodynamic efficiency is 55.7% and 65.7% for both H2O splitting and CO2 splitting at 2000 °C. An efficient O2 separation method is extremely crucial to achieve high thermodynamic efficiency, especially in the separation efficiency range of 0–20% and in relatively low reaction temperatures. This research is also applicable in other isothermal H2O or CO2 splitting systems (e.g., chemical cycling) due to similar thermodynamics.

Four different nanoshapes of cerium dioxide have been prepared (polycrystals, rods, cubes, and octahedra) and have been decorated with different metals (Ru, Pd, Au, Pt, Cu, and Ni) by incipient wetness impregnation (IWI) and ball milling (BM) methods. After an initial analysis based on oxygen consumption from CO2 pulse chemisorption, Ni-like metal, and two forms of CeO2 cubes and rods were selected for further research. Catalysts were characterized using the Brunauer-Emmett-Teller formula (BET), X-ray spectroscopy (XRD), Raman spectroscopy, scanning electron microscopy (SEM), UV–visible spectrophotometry (UV-Vis), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (H2-TPR) and CO2 pulse chemisorption, and used to reduce of CO2 into CO (CO2 splitting). Adding metals to cerium dioxide enhanced the ability of CeO2 to release oxygen and concomitant reactivity toward the reduction of CO2. The effect of the metal precursor and concentration were evaluated. The highest CO2 splitting value was achieved for 2% Ni/CeO2-rods prepared by ball milling using Ni nitrate (412 µmol/gcat) and the H2 consumption (453.2 µmol/gcat) confirms the good redox ability of this catalyst.

Metal oxide (MO) based solar thermochemical H2O (WS) and CO2 splitting (CDS) is one of the most promising and potential-containing processes that can be used to produce H2 and syngas (liquid fuel precursor). Several non-volatile and volatile MOs were considered redox materials for the solar-driven WS and CDS operation. Among all the examined redox materials, based on their high O2 storage capacity, faster oxidation kinetics, and good stability, ceria and doped ceria materials are deemed to be one of the best alternatives for the operation of the thermochemical redox reactions associated with the WS and CDS. Pure ceria was used for solar fuel production for the first time in 2006. A review paper highlighting the work done on the ceria-based solar thermochemical redox WS and CDS cycle from 2006 until 2016 is already published elsewhere by the author. This review paper presents all the significant findings reported in applying pure ceria and doped ceria materials for the WS and CDS by research teams worldwide.

By employing metal oxides as oxygen carriers, chemical looping demonstrates its effectiveness in transferring oxygen between reduction and oxidation environments to partially oxidize fuels into syngas and convert CO2 into CO. Generally, NiFe2O4 oxygen carriers have demonstrated remarkable efficiency in chemical looping CO2 conversion. Nevertheless, the intricate process of atomic migration and evolution within the internal structure of bimetallic oxygen carriers during continuous high‐temperature redox cycling remains unclear. Consequently, the lack of a fundamental understanding of the complex ionic migration and oxygen transfer associated with energy conversion processes hampers the design of high‐performance oxygen carriers. Thus, in this study, we employed in situ characterization techniques and theoretical calculations to investigate the ion migration behavior and structural evolution in the bulk of NiFe2O4 oxygen carriers during H2 reduction and CO2/lab air oxidation cycles. We discovered that during the H2 reduction step, lattice oxygen rapidly migrates to vacancy layers to replenish consumed active oxygen species, while Ni leaches from the material and migrates to the surface. During the CO2 splitting step, Ni migrates toward the core of the bimetallic oxygen carrier, forming Fe–Ni alloys. During the air oxidation step, Fe–Ni migrates outward, creating a hollow structure owing to the Kirkendall effect triggered by the swift transfer of lattice oxygen. The metal atom migration paths depend on the oxygen transfer rates. These discoveries highlight the significance of regulating the release–recovery rate of lattice oxygen to uphold the structures and reactivity of oxygen carriers. This work offers a comprehensive understanding of the oxidation/reduction‐driven atomic interdiffusion behavior of bimetallic oxygen carriers. During chemical looping CO2 conversion, in situ environmental transmission electron microscopy–electron energy loss spectroscopy combined with quasi in situ X‐ray photoelectron spectroscopy, and theoretical calculations have been utilized to reveal the migration and diffusion processes of lattice oxygen and metal atoms inside oxygen carriers. It is highlighted that the structure of the oxygen carrier depends on the migration rate of lattice oxygen.

A thermodynamic efficiency analysis of MnFe2O4-based CO2 splitting (CDS) cycle is reported. HSC Chemistry software is used for performing the calculations allied with the model developed. By maintaining the reduction nonstoichiometry equal to 0.1, variations in the thermal energy required to drive the cycle and solar-to-fuel energy conversion efficiency as a function of the ratio of the molar flow rate of inert sweep gas to the molar flow rate of Mn-ferrite, reduction temperature, and gas-to-gas heat recovery effectiveness are studied. This study confirms that the thermal reduction temperature needed to achieve reduction nonstoichiometry equal to 0.1 is reduced when the inert gas flow rate is increased. Conversely, due to the requirement of the additional energy to heat the inert gas, the thermal energy required to drive the cycle is upsurged considerably. As the solar-to-fuel energy conversion efficiency depends significantly on the thermal energy required to drive the cycle, a reduction in it is recorded. As the ratio of the molar flow rate of inert sweep gas to the molar flow rate of Mn-ferrite is increased from 10 to 100, the solar-to-fuel energy conversion efficiency is decreased from 14.9% to 9.9%. By incorporating gas-to-gas heat recovery, a drastic drop in the thermal energy required to drive the cycle is attained which further resulted in a rise in the solar-to-fuel energy conversion efficiency. The maximum solar-to-fuel energy conversion efficiency (17.5%) is achieved at the ratio of the molar flow rate of inert sweep gas to the molar flow rate of Mn-ferrite equal to 10 as well as 20 when 90% of gas-to-gas heat recovery is applied.

ABSTRACT Advanced oxygen carrier plays a pivotal role in various chemical looping processes, such as CO2 splitting. However, oxygen carriers have been restricted by deactivation and inferior oxygen transferability at low temperatures. Herein, we design an Fe–Ov–Ce–triggered phase‐reversible CeO2−x·Fe·CaO ↔ CeO2·Ca2Fe2O5 oxygen carrier with strong electron‐donating ability, which activates CO2 at low temperatures and promotes oxygen transformation. Results reveal that the maximum CO2 conversion and CO yield obtained with 50 mol% CeO2−x·Fe·CaO are, respectively, 426% and 53.6 times higher than those of Fe·CaO at 700°C. This unique multiphase material also retains exceptional redox durability, with no obvious deactivation after 100 splitting cycles. The addition of Ce promotes the formation of the Fe–Ov–Ce structure, which acts as an activator, triggers CO2 splitting, and lowers the energy barrier of C═O dissociation. The metallic Fe plays a role in consuming O2−lattice transformed from Fe–Ov–Ce, whereas CaO acts as a structure promoter that enables phase‐reversible Fe0 ↔ Fe3+ looping. A phase‐reversible CeO2‐x·Fe·CaO↔CeO2·Ca2Fe2O5 oxygen carrier is designed and synthesized with enhanced CO2‐splitting capability and robust redox durability. The engineered Fe–Ov–Ce structure acts as an initiator to induce CO₂ activation and dissociation, Fe acts as an active phase to accommodate oxygen ions from CO₂ dissociation, and CaO functions as a structural promoter, enhancing deep oxidation of Fe° into Fe3+.

A two-stage catalytic membrane reactor (CMR) that couples CO2 splitting with methane oxidation reactions was constructed based on an oxygen-permeable perovskite asymmetric membrane. The asymmetric membrane comprises a dense SrFe0.9Ta0.1O3-δ (SFT) separation layer and a porous Sr0.9(Fe0.9Ta0.1)0.9Cu0.1O3-δ (SFTC) catalytic layer. In the first stage reactor, a CO2 splitting reaction (CDS: 2CO2 → 2CO + O2) occurs at the SFTC catalytic layer. Subsequently, the O2 product is selectively extracted through the SFT separation layer to the permeated side for the methane combustion reaction (MCR), which provides an extremely low oxygen partial pressure to enhance the oxygen extraction. In the second stage, a Sr0.9(Fe0.9Ta0.1)0.9Ni0.1O3-δ (SFTN) catalyst is employed to reform the products derived from MCR. The two-stage CMR design results in a remarkable 35.4% CO2 conversion for CDS at 900 °C. The two-stage CMR was extended to a hollow fiber configuration combining with solar irradiation. The solar-assisted two-stage CMR can operate stably for over 50 h with a high hydrogen yield of 18.1 mL min−1 cm−2. These results provide a novel strategy for reducing CO2 emissions, suggesting potential avenues for the design of the high-performance CMRs and catalysts based on perovskite oxides in the future. [Display omitted] •Based on SFT perovskite, SFTC/SFT asymmetric membranes were successfully fabricated.•A novel two-stage catalytic membrane reactor coupling CO2 splitting and methane oxidation was constructed.•The CO2 conversion reaches 35.4% with a hydrogen yield of 18.1 mL min−1 cm−2.•The two-stage CMR can extend to hollow fiber configuration.•The solar-assisted membrane reactor can operate stably for 50 h without any degradation.

In this investigation, an effort was made to introduce Ag into the CeO2 fluorite crystal lattice to form Ce0.99Ag0.01O2-δ (ACRM) using an ammonium hydroxide-assisted co-precipitation method. The resulting powder obtained after the co-precipitation reaction, filtration, and drying was annealed at 800 °C in a muffle furnace to obtain crystalline ACRM. The phase composition and microstructure of the synthesized ACRM were analyzed using a powder X-ray diffractometer (PXRD) and a scanning electron microscope (SEM). The characterized ACRM powder was then subjected to multiple thermochemical thermal reduction (TR) and CO2 splitting (CDS) cycles using a high-temperature thermogravimetric analyzer (TGA). The TR step was conducted using Ar gas as an inert atmosphere, maintaining the temperature at 1400 °C for 60 min. Subsequently, the same powder was subjected to the CDS step by treating it with a gaseous mixture of 50% CO2 and Ar gas at 1000 °C for 30 min. ACRM displayed stable redox reactivity towards thermochemical CDS cycles by generating an average of 50.9 μmol of O2/g·cycle and 101.6 μmol of CO/g·cycle, respectively, over 10 thermochemical cycles.

The present study provides a thermodynamic analysis of solar thermochemical cycles for splitting of H2O or CO2. Such cycles, powered by concentrated solar energy, have the potential to produce fuels in a sustainable way. We extend a previous study on the thermodynamics of water splitting by also taking into account CO2 splitting and the influence of the solar absorption efficiency. Based on this purely thermodynamic approach, efficiency trends are discussed. The comprehensive and vivid representation in T-S diagrams provides researchers in this field with the required theoretical background to improve process development. Furthermore, results about the required entropy change in the used redox materials can be used as a guideline for material developers. The results show that CO2 splitting is advantageous at higher temperature levels, while water splitting is more feasible at lower temperature levels, as it benefits from a great entropy change during the splitting step.

Highlights BiOCl atomic layers (BOCNSs-i) were prepared by exfoliating hydrothermally synthesized BiOCl (BOCNSs) via ultrasonication in isopropanol for efficient photocatalytic CO 2 overall splitting to CO and O 2 . The obtained BOCNSs-i photocatalyst exhibits a distinctly improved photocatalytic performance to stoichiometrically produce CO and O 2 at the ratio of 2:1, with the CO evolution rate reaching 134.8 µmol g −1  h −1 under simulated solar light (1.7 suns) and reaching 13.3 mmol g −1  h −1 under concentrated solar irradiation (34 suns). With the thickness of BiOCl photocatalyst reducing to atomic layers, the charge carrier transfer and separation were enhanced by shortened transfer distance and the increased built-in electric field intensity, and electrons enriched Bi sites were exposed for activating CO 2 molecules. Given the limited exposure of active sites and the retarded separation of photogenerated charge carriers in those developed photocatalysts, photocatalytic CO 2 splitting into value-added chemicals has suffered from the poor activity and remained in great challenge for real application. Herein, hydrothermally synthesized BiOCl with layered structure (BOCNSs) was exfoliated into thickness reduced nanosheets (BOCNSs-w) and even atomic layers (BOCNSs-i) via ultrasonication in water and isopropanol, respectively. In comparison with the pristine BOCNSs, the exfoliated BiOCl, especially BOCNSs-i with atomically layered structure, exhibits much improved photocatalytic activity for CO 2 overall splitting to produce CO and O 2 at a stoichiometric ratio of 2:1, with CO evolution rate reaching 134.8 µmol g −1  h −1 under simulated solar light (1.7 suns). By surpassing the photocatalytic performances of the state-of-the-art Bi l O m X n (X: Cl, Br, I) based photocatalysts, the CO evolution rate is further increased by 99 times, reaching 13.3 mmol g −1  h −1 under concentrated solar irradiation (34 suns). This excellent photocatalytic performance achieved over BOCNSs-i should be benefited from the shortened transfer distance and the increased built-in electric field intensity, which accelerates the migration of photogenerated charge carriers to surface. Moreover, with oxygen vacancies (V O ) introduced into the atomic layers, BOCNSs-i is exposed with the electrons enriched Bi active sites that could transfer electrons to activate CO 2 molecules for highly efficient and selective CO production, by lowering the energy barrier of rate-determining step (RDS), *OH + *CO 2 −  → HCO 3 − . It is also realized that the H 2 O vapor supplied during photocatalytic reaction would exchange oxygen atoms with CO 2 , which could alter the reaction pathways and further reduce the energy barrier of RDS, contributing to the dramatically improved photocatalytic performance for CO 2 overall splitting to CO and O 2 .